There is a tendency that the amount of heat generated by semiconductor devices increases with improvement in the functionality and performance of electronic equipment. In particular, power semiconductor devices for power and supply control are required to achieve high-voltage and large-current characteristics. Devices having mounted thereon silicon carbide (SiC) semiconductor elements, gallium nitride (GaN) semiconductor elements etc. are expected as the next-generation power semiconductor devices. It is assumed that the full potential of these devices are exploited by stable operations at about 250° C. Under such circumstances, the heat-resistant temperature required of peripheral components such as encapsulants are becoming high.
Conventionally, epoxy resin compositions are used as raw materials of encapsulants for power semiconductor elements. It is generally known that cured products of epoxy resin compositions, when left for a long time under temperature conditions of 150° C. or higher, show deteriorations in weight and mechanical strength. Various studies have thus been made to improve the heat resistance of cured epoxy resin products. For example, Japanese Laid-Open Patent Application Publication No. 2014-9936 discloses an epoxy resin-containing cured product applicable up to temperatures near 180° C. However, the heat resistance of this cured product is not yet sufficient as required for the operations of SiC and GaN power semiconductor devices at about 250° C.
Silicone resin compositions are also widely used as encapsulants for power semiconductor elements. As methods for obtaining cured products of silicon resin compositions, there are known a method using hydrosilylation between a hydrosilyl group and an alkenyl group (see, for example, Japanese Laid-Open Patent Application Publication No. 2008-27966 and No. 2005-146191), a method using polymerization reaction by a reactive functional group such as epoxy group (see, for example, International Patent Application Publication No. 2004/072150) and the like. In these methods, however, the cross linkages of the cured products are poor in thermal stability so that the cured products are not always suitable for uses where the cured products are required to have heat resistance at about 250° C. for a long time.
As another curing method, there is also known a method using so-called condensation and, more specifically, at least one of dehydration condensation between silanol groups, dealcoholization condensation between a silanol group and an alkoxysilyl group and dehydrocondensation of a silanol group and a hydrosilyl group. Polysiloxane compounds usable as raw materials for the production of cured products by such a condensation curing method are called condensation type polysiloxane compounds. It is known that cured products of condensation type polysiloxane compounds have both main chain and cross-linkage structures constituted only by chemically stable siloxane bond and thus show very high heat resistance. Further, the condensation curing method enables forming and curing at temperatures of 200° C. or lower and thus can suitably be utilized for encapsulation of semiconductor elements with heat-sensitive components. It is consequently possible to attain high material selectivity by the condensation curing of condensation type polysiloxane compounds as compared to the case of using the other heat-resistant materials such as ordinary polyimide resin and molten glass that need to be subjected to forming and curing at temperatures exceeding 200° C. However, the condensation curing method has the problem of the occurrence of foaming in the cured product by generation of gas (water, alcohol and/or hydrogen) during curing. The foaming is regarded as a problem since the occurrence of foaming leads to deteriorations in the adhesion, mechanical strength, gas barrier function and insulating property of the cured product. It is essentially difficult to solve such a foaming problem due to the nature of the condensation reaction.
Various studies have been made to suppress the occurrence of foaming in cured products of condensation type polysiloxane compositions. For example, Japanese Laid-Open Patent Application Publication No. 2009-256670 discloses a condensation type polysiloxane composition capable of being formed into a cured product with an average thickness of 1.2 mm or smaller. It is discussed in this patent publication that the occurrence of foaming in the cured product can be reduced by decreasing the film thickness of the composition. Furthermore, Japanese Laid-Open Patent Application Publication No. 2011-219729 discloses a condensation type polysiloxane composition containing polydimethylsiloxane having two silanol groups bonded to both terminal ends thereof. In this composition, the condensation site of the polysiloxane is decreased so as to control the thickness of the cured product to 1 mm or smaller reducing the amount of gas generation during curing.